promise

What it is and what it does

Promise is a Python implementation of the Promises/A+ specification, a standard for handling asynchronous operations through chainable callbacks. It lets you wrap async work in a Promise object, resolve or reject it with a value, and attach handlers via .then() and .catch() that execute when the promise settles. The library supports composing multiple promises (Promise.all, Promise.for_dict) and wrapping existing futures.

The package has no runtime dependencies and works with Python 2.7 through 3.8 (and PyPy). It is designed to be a readable, performant alternative to raw callbacks for async patterns in Python, though modern Python now has native async/await syntax and asyncio, which may be preferable for new code.

Use it for:

• Wrapping callback-based async operations (e.g., I/O) in a chainable Promise interface for cleaner control flow
• Composing multiple async tasks with Promise.all() to wait for all to complete before proceeding
• Converting Python Futures into Promises for interoperability with promise-based libraries or frameworks
• Building promise-based libraries or frameworks that need a standard async abstraction layer
• Handling rejection and error propagation through .catch() in async workflows without explicit try/except nesting

Worth the install?

AI-flagged interpretation of the facts on this page — verify before relying

Implements Promises/A+ specification for Python, enabling asynchronous value resolution and rejection with chainable then/catch handlers.

Yes-with-conditions. The package is stable (MIT, no known vulnerabilities, 367 GitHub stars) but dormant since 2019. Install only if you are maintaining legacy code that already uses promise or integrating with a framework that requires it. For new projects, use Python's native async/await and asyncio instead.

Install

promise on PyPI

pip install promise

uv add promise

poetry add promise

Installing promise

Before you install

High install friction due to source-only distribution (promise-2.3.tar.gz). Maintenance is dormant—last release was 2019-12-18 and last commit 2024-07-09, with no active development signals.

License in practice

MIT license (permissive) places no restrictions on use, modification, or distribution. Safe for commercial and open-source projects.

Quickstart

from promise import Promise

promise = Promise(lambda resolve, reject: resolve('RESOLVED!'))
result = promise.then(lambda value: print(value))

Verify before relying

• Whether promise.get() method exists and how it blocks/retrieves resolved values (shown in docs but not formally specified)
• Python 3.9+ compatibility—classifiers list only up to 3.8; current support unclear
• Whether this package is maintained or if a modern alternative is recommended for new projects

Package facts

Evidence: promise-2.3.tar.gz

Keywords: concurrent, future, deferred, promise

Tags

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